Introduction of DNA into an animal for the purpose of eliciting an immune response is often referred to as DNA vaccination. DNA vaccination represents a means of expressing an antigen in vivo for the generation of humoral and cellular immune responses. DNA vaccines employ genes encoding antigens, rather than using the proteins themselves, to induce immune responses. The DNA, upon administration to the host, is transcribed and translated in vivo to produce an antigen. Processing and presentation of the antigen stimulates the animal's immune system to elicit a humoral and/or cellular response to the antigen. This immune response can potentially confer protective immunity to the animal.
DNA vaccines appear to have advantages over protein antigen-based vaccines, standard “killed” pathogen vaccines, live, attenuated vaccines, and recombinant viral vector vaccines. For example, DNA vaccines appear to be more effective in producing an antigen with a properly folded, native three-dimensional conformation and in generating a cellular immune response than are protein antigens. DNA vaccines also do not exhibit at least some of the safety problems of killed, live or virally-vectored vaccines. For example, a killed virus preparation may contain residual live viruses or may need to be mixed with reactogenic adjuvants, such as those associated with vaccine-related fibrosarcomas in cats, in order to stimulate an effective immune response. An attenuated virus may mutate and revert to a pathogenic phenotype. Viral vector vaccines genetically engineered to express a gene encoding the desired antigen may stimulate the production of antibodies that react with the virus as well; such antibodies may render futile any further attempt to use that virus as a vector, even with a different gene insert. In contrast, DNA vaccines apparently are non-reactogenic and, if they elicit an immune response, that response is targeted against the antigen of choice.
DNA vaccines typically include a bacterial plasmid, a strong viral promoter, the gene of interest, and a polyadenylation/transcriptional termination sequence. The plasmid is typically grown in bacteria, purified, dissolved in a saline solution, and then simply injected into an animal. Current understanding of how to use DNA vaccines to generate an effective immune response, however, is not complete. Most of our understanding of the mechanisms of DNA vaccine action is derived from rodent studies. In mice, bone marrow-derived antigen-presenting cells have been shown to induce cytotoxic T lymphocyte responses following intramuscular inoculation of naked plasmid DNA. In some cases, DNA vaccination has also been shown to stimulate antigen-specific antibodies, some of which may be neutralizing antibodies. DNA vaccines have also been administered to large animals, albeit with varying degrees of success. While there are some clear examples of DNA vaccine efficacy in large animals, other studies cite relatively weak responses, requirement for large amounts of DNA, or the need for multiple immunizations. As such, it is apparent that further technology development is required to maximize DNA vaccine efficacy in humans and large animals.
Immune responses to DNA vaccination appear to vary according to the vehicle used with the DNA vaccine, the antigen expressed by the DNA vaccine, the route of administration, and the species of mammal into which the DNA vaccine is injected. Investigators have used different vehicles and/or genes encoding cytokines and other stimulatory molecules in an attempt to enhance the immune response to the antigens encoded by DNA vaccines with mixed success. Although cationic lipids have been used to deliver nucleic acids to cells in vitro and in vivo, there is no consensus in the literature about whether cationic lipids reproducibly enhance the immunogenicity of DNA vaccines. Gregoriadis et al., 1997, FEBS Letters 402, 107, reported that intramuscular (I.M.) injection of DNA encoding HBsA “entrapped” in cationic liposomes into mice elicited an enhanced immune response compared to I.M. injection of “naked” DNA encoding HBsA, whereas DNA encoding HBsA merely “complexed” with cationic lipid generated a reduced immune response compared to “naked” DNA. Ishii et al., 1997, AIDS Research and Human Retroviruses 13, 1421-1424, demonstrated enhanced immune responses to V3 peptide following I.M., intraperitoneal (I.P.), intradermal (I.D.), intranasal (I.N.) or subcutaneous (S.Q.) administration to mice.
Other investigators, in contrast, found no enhancement of immune responses when cationic lipids were used as a vehicle for DNA vaccines in mice. For example, Davis, et al., 1997, Vaccine 15, 849, found that DNA vaccines encoding the Hepatitis B surface antigen formulated with varying amounts of cationic lipids performed no better than DNA alone in inducing a humoral response in mice. Gramzinski, et al., 1998, Molecular Medicine 4, 109, reported that Aotus monkeys administered DNA vaccines encoding HBsA either with or without cationic lipids (CELLFECTIN®, 10:1 DNA:lipid) by I.M. injection did not seroconvert. Clearly, there is no consensus regarding whether cationic lipids reproducibly act to elicit or enhance immune responses to DNA vaccines.
There also appears to be a high degree of variability of the efficacy of DNA vaccines between different routes of administration. Ishii et al, ibid., for example, found in mice that I.M. and I.N. administration of DNA vaccines generated approximately equivalent immune responses, but that I.P. administration was less effective, and that I.D. and S.Q. administration routes were even less effective. Ishii et al, ibid., found these differences to be consistent regardless of whether DNA was used alone or formulated with cationic lipids. Yokoyama et al, 1996, FEMS Immuno Med Microbio 14, 221-230, showed that I.V. administration of a DNA vaccine generated a better immune response than I.M. administration of the same vaccine in mice.
Taken together, these data indicate that there is a high degree of variability in the effectiveness of DNA vaccines and in the ability of cationic lipids to enhance the effectiveness of DNA vaccines both within and between species and routes of administration.
There are a number of diseases in cats which lead to significant morbidity and mortality. It would be desirable to provide novel and safe vaccines that would confer protective immunity to these diseases. That there is still a need for such vaccines is underscored not only by the association of some feline vaccines with the development of fibrosarcomas but also by the finding that I.M. administration of naked DNA encoding either human growth hormone (hGH) or rabies virus glycoprotein G into domestic cats resulted in incomplete seroconversion, even after two immunizations (Osorio et al, 1999, Vaccine, in press). These results indicate that parenteral naked DNA vaccination efficacy in cats is inferior to results obtained in mice, and that the efficacy achieved using naked DNA in cats is not sufficient to protect cats from disease. Thus, there remains a need to provide a method to elicit and to enhance the immune response to antigen encoded by DNA vaccines in cats.